38 THE ROMAN COMAGMATIC REGION. 
be definitely ascertained, as most of the leucites were much broken in the sections 
and their inclusions lost. Of the two modes, that calculated from the norm is 
regarded as the more reliable and the nearer approximation to the true composition. 
From these it is clear that the modal deviations from the norm due to alferric 
minerals are inconsiderable, but that the essential difference lies in the modal 
leucite replacing a negligible amount of normative nephelite. The relations of mode 
and norm are of especial interest as illustrative of the possibility that a rock 
belonging to a perfelic order may carry modally about 30 per cent of the lenad 
leucite. This will be adverted to in a later discussion. From the description of 
the rock and the relation of the mode to the norm, the type may be described as a 
leucite-salphyro-vulsinose. 
The mode of II was less satisfactorily estimated by Rosiwal's method, and as 
thus determined agrees far less closely with that calculated from the norm. The 
leucite phenocrysts were determined by measurements on the surfaces of the hand 
specimen, and those of feldspar and augite phenocrysts in thin section, both as 
compared with the groundmass as a whole. Attempts to estimate the relative 
amounts of the minerals in this last were unsatisfactory, because of their very small 
size and the consequent large error due to overlapping. As thus determined the 
mode may be stated as follows: 
Vol. % Sp. gr. Wt. % 
Orthoclase phenocrysts 1.1X2.6= 2.9 i.i 
Labradorite phenocrysts .... 14.1 X 2.7 = 38.1 14.7 
Leucite phenocrysts 38.1 X 2.5 = 95-3 36.8 
Augite phenocrysts 1.4 X 3.3 = 4.6 1.8 
Groundmass 45-3 X 2.6 = 117.8 45-6 
100.0 258.7 100.0 
Comparing this with the Rosiwal mode of the Grignano rock, it is seen that the 
amounts of leucite phenocrysts in both are almost alike, and as these could be 
measured with considerable accuracy, it must be assumed that the measured amounts 
of this mineral are very close to the truth, though slight corrections must be made 
for the small quantities of inclusions which the large phenocrysts carry, and which 
it was not practicable to estimate. 
The mode was also calculated from the norm by the usual process of successive 
readjustments, all the normative olivine being changed to hypersthene, which enters 
into modal augite, and the normative nephelite taking silica enough to form albite, 
which enters into modal alkali- feldspar and labradorite. In this way we obtain 
the maximum amount of molecules of minerals as highly silicated as possible, leav- 
ing as little silica as possible to combine with potash in orthoclase and leucite, the 
amounts of which are thus theoretically fixed and can be calculated by the equa- 
tions given in the "Quantitative Classification" on page 194. We must thus 
obtain the maximum amount of leucite capable of formation from the magma, 
as, if more were assumed to be present, some silica would necessarily be set free, 
which is contrary to the actual facts. 
